Fig. 1. Microtubule cytoskeleton in a migrating Swiss 3T3 fibroblast visualized by the microinjection of X-rhodamine-conjugated tubulin and fluorescence microscopy. The contrast of the images was inverted to show individual microtubules more clearly. Note how microtubules are aligned along the axis of migration and how growing microtubules fill in the protruding leading edge as the cell moves forward.